Diversity rf alarm system

ABSTRACT

An alarm system incorporating radio links between a plurality of remotely located alarm condition sensing units and a central receiving station. Each alarm condition sensing unit is comprised of at least one sensing means which is comprised of an alarm sensing element in combination with a frequency diversity radio transmitter means. The transmitter means of all of the sensing units transmit frequency diversity carrier signals having the same carrier frequencies but the carrier signals associated with different units have modulation signals of differenct frequency impressed thereon. At the receiving station a frequency diversity receiver means which has two channels and a means for switching therebetween demodulates the frequency diversity signals received. The demodulated signals pass to a plurality of pulse rate discriminators equal in number to the number of sensing units in the system. Each pulse rate discriminator is responsive to a different modulation signal to emit a different alarm output which alarm output indicates the sensing unit which has triggered. Each pulse rate discriminator means indicates whether a given modulation frequency is present or not by comparing the time between successive negative going zero crossings of the demodulated signal with the duration of a plurality of fixed length pulses.

United States Patent 1 Birchfield et al. Sept. 4, 1973 DIVERSITY RFALARM SYSTEM [57] ABSTRACT [75] Inventors: Jerry Linward Birchfield,Decatur' An alarm s ystem incorporating radio links between a g a ggMoss Marlena plurality of remotely located alarm condition sensing t ounits and a central receiving station. Each alarm condi- [73] Assignee:Rollins Protective Services Company, sensing unit is comprised of atleast one Sensing Atlanta, Ga. means which is comprised of an alarmsensing element in combination with a frequency diversity radio trans-[22] 1971 mitter means. The transmitter means of all of the sens- .[2l.]Appl. No.; 176,058 ing units transmit frequency diversity carriersignals having the same carrier frequencies but the carrier signalsassociated with different units have modulation 2? 340/2204 325/56signals of differenct frequency impressed thereon. At i h l 7/02 G 8b21/00 the receiving station a frequency diversity receiver 1 325/154 1means which has two channels and a means for switchl 1 1 1797/ ingtherebetween demodulates the frequency diversity v, 40/2 181 signalsreceived. The demodulated signals pass to a pluv rality of pulse ratediscriminators equal in number to [56'] References cued" the number ofsensing units in the system. Each pulse 7 V UNITED STATES PATENTS 7 ratediscriminator is responsive to a different modula- 3,149,317 9/1964Brugliera 340/224 tion signal to emit a different alarm outputwhichalarm 3,114,106 12/1963 McManu s... 325/56 output indicates the sensingunit which has triggered. 31301454 1/1966 k 340/224 X Each pulse ratediscriminator means indicates whether 3,310,742 3/1967 Adams 325/56 agiven moduhtion fr uency i e ent or not by comparing the time betweensuccessive negative going zero rec crossings of the demodulated signalwith the duration Primary Examiner ponald J. Yusko of a plurality offixed length pulses. Attorney-Beveridge & DeGrandi 22 Claims, 6 DrawingFigures HEAT i OSCILLATOR I CHANNEL A I 25 I 3 l I l 22 l O T RF MIXERDEMODULATOR W 20 T AMP I l L l 2| I BAND POWER 10.7 MHZ IF PASS r HUERSWITCH FILTER AMPLIFIER L 29 "l l l RF I 1 AMP I CHANNEL 3 1 l I l 1PAIENTEDSEP 41915 5.757. 315

SHEEI 1 RE 3 K In lb lc SENSING UNIT 0 O O DECODERI RECEIVER 20 2b 2cPRESELECTOR SENSING uvn 2 O O O I DECODER 2 v 3a 3b 3c SENSING UNIT 3 0O 0 DECODER 3 I' v 4a 4b 4c SENSING UNIT 4 O O O DECODER 4 RECEIVER 505b 5c PRESELECTOR SENSING UNIT 5 O O O DECODER 5 6a 6b 6c SENSING mm; 0O O DECODER s 295.7 30o 304.? MHZ) INVENTOR "(5. l JERRY L. BIRCHFIELDnumb uJ- M 55 BY f 3 4 ra-q ATTORNEYS BAND PASS

- FILTER CA I E E PE PAnmm E 3.757.

saw Mr 3 I 3 RELAXATION SUBCARRIER OSCILLATOR OSCILLATOR CRYSTAL x 3BALANCED OSCILLATOR MULTIPLIER MODULATOR DEMODULATOR T0 DECODERAMPLIFIER PAIENTEDSEP w 3.157. 315

sum 3 or 3 RESETTABLE 4 PULSE 5 FROM RECEIVER BANDPASS SCHMITT 2 'PULSE3 DELAYO/S 0/5 ETLTER TRIGGER SHAPER 5 45 AND PULSE SCHMITT CHARGINGGATED PULSE GATE couNTER LEVEL DET. NETWORK GENERATOR T 4s DUMP 4T 49Z50 OUTPUT RESET 48 0/8 FIG. 5

U k/ l l r T l H6. 6 T {2 A (L A 3 T--.9T

DIVERSITY RF ALARM SYSTEM This invention relates to an alarm systemutilizing radio links between a plurality of remotely located conditionsensing units and a central receiving station and has the ability toindicate at the central receiving station which of the plurality ofremotely located sensing units has triggered. The alarm system accordingto the present invention is easily expanded or contracted to includemore or less remote sensing locations and is extremely reliable and ishighly immune to false alarms.

Residential and industrial alarm systems can employ several methods forrelaying the alarm signal from an alarm sensing unit to an alarmactuator. One method is to install wires over which the alarm signal canbe transmitted. Another method is to transmit the alarm signal overpower lines already present in the installation. Alarm systems usingthese methods of relaying the alarm signal are undesirable because theycannot easily be expanded or contracted to encompass more or less remotesensing locations and are therefore not flexible. Additionally, there isa great amount of labor involved if an installation is to be wired.

The above problems can be obviated by the use of an alarm system whichincorporates radio alarm links. The prior art alarm systems, however,which have utilized radio links have been unsuitable because they havenot been sufficiently reliable and have been subject to false alarms.With a radio alarm link it is of critical importance to eliminateinterference from nearby electromagnetic sources such as commercialradio and television stations as well as from motors and other sourcesof power. Also the alarm system should not be triggered by the harmonicsof the alarm signal itself and a radio alarm transmission link must becompletely reliable and must not be dependent upon weather conditions orsignal fading. An alarm system using radio links should additionally becapable of being used in conjunction with other similar alarm systems inthe same vicinity without the alarm systems interfering with each other.

The alarm system of the present invention overcomes the disadvantages ofthe prior art alarm systems by employing a plurality of frequencydiversity radio transmitter means in conjunction with alarm conditionsensing elements at remote locations and a single frequency diversityreceiver means at a central location. The frequency diversity radiolinks are extremely reliable and overcome the problems of multipathfading and cancellation which have seriously hampered the performance ofprior art radio alarm systems.

Additionally, the alarm system of the present invention overcomes theproblem of false alarms triggered by spurious signals by the use of aplurality of pulse rate discriminator means at the central receivingstation to respond to the different modulation signals of thetransmitters associated with different sensing units. The pulse ratediscriminator means indicate whether a given frequency is present bycomparing the time between negative going zero crossings of the inputwave to the discriminator with the duration of a plurality of fixedlength pulses generated by pulse generating means. The pulse ratediscriminator means are therefore responsive only to the frequency ofthe demodulated radio signals and not to the amplitude of thedemodulated radio signals or to the amplitude of any spurious signals,and therefore cannot be triggered by intcrfercncc signals or byharmonics of the alarm signal itself.

It is therefore an object of the invention to provide an alarm systemincorporating radio links between a plurality of alann condition sensingunits and a central receiving station which is reliable and which is notsubject to false alarms.

It is a further object of the invention to provide an alarm systemincorporating radio links between a plurality of alarm condition sensingunits and a central receiving station which is not subject to signalfading.

It is a further object of the invention to provide an alarm systemutilizing radio links between aplurality of remotely located alarmcondition sensing units and a central receiving station which has theability to indicate at the central receiving station which of theplurality of remotely located sensing units has triggered.

It is a further object of the invention to provide an alarm systemutilizing radio links between a plurality of remotely located alarmcondition sensing units and a central receiving station wherein aplurality of pulse rate discriminators are used at the central receivingstation which are responsive only to the frequency of the demodulatedsignal and not to amplitude to identify the radio signals transmitted byparticular sensing units.

It is a further object of the invention to provide a plurality of alarmsystems operating within the same vicinity wherein each alarm systemcomprises a plurality of alarm condition sensing units operating inconjunction with a central receiving station and wherein said pluralityof alarm systems do not interfere with each other.

It is also an object of the invention to provide an improved pulse ratediscriminator means which generates an output signal only when a signalof predetermined frequency is present at its input and which isresponsive only to the frequency of the input signal and not to theamplitude.

The present invention accomplishes the above objects by providing aplurality of alarm sensing units at remote locations and a receivermeans at a central location. Each sensing unit is comprised of at leastone sensing means which comprises an alarm condition sensing elementconnected to a frequency diversity radio transmitter means which isoperative to transmit a pair of frequency diversity signals displaced infrequency when triggered by the sensing element to which it isconnected. The transmitter means are amplitude modulated and alltransmitter means associated with a particular receiving locationtransmit frequency diversity signals having the same carrierfrequencies. The carrier signals of transmitters of different units havemodulation signals of different frequency impressed thereon.

The frequency diversity receiver means at the central location is asuperhetrodyne type receiver which is tuned to the carrier frequenciesof the frequency diversity signals. The frequency diversity signals aredemodulated in the receiver means and trigger one of a plurality ofpulse rate discriminator means depending on the modulation frequency ofthe demodulated signal. Each pulse rate discriminator means is a noveldevice for indicating whether a signal of predetermined frequency ispresent at its input terminals and works by comparing the time periodbetween negative going zero crossings of the input wave with theduration of a plurality of fixed length pulses generated by pulsegenerating means.

Each frequency diversity transmitter means of the invention includes acrystal oscillator and a times three multiplier for generating a centercarrier frequency, a subcarrier oscillator for generating a subcarrierfrequency, and a balanced modulator for mixing the center carrierfrequency with the subcarrier frequency for generating two side bandsignals displaced in the frequency spectrum. A relaxation oscillatormodulates the subcarrier oscillator with an ultrasonic modulationsignal.

The frequency diversity receiver means of the invention is asuperhetrodyne type receiver employing dual channels with means forswitching therebetween. Each channel includes a radio frequencyamplifier, a local oscillator, and a mixer. Switching between the twochannels is done at a low frequency rate and the outputs of bothchannels are connected to an intermediate frequency filter andintermediate frequency amplifier of narrow bandwidth. After passingthrough the intermediate frequency amplifier the signals are demodulatedand passed to the decoder means of the invention which comprises aplurality of pulse rate discriminators. Additionally, a preselectorfilter is located at the input to the receiver means for rejectingsignals outside of a predetermined band.

Each pulse rate discriminator means of the invention comprises a meansfor generating pulses at each negative going zero axis crossing of thedemodulated wave input to the discriminator, said pulse generatinghaving two outputs, one output of which is connected to an AND gate andthe other output of which is connected to two one shot multivibrators inseries. The output of the second one shot multivibrator is alsoconnected to the input of the AND gate. The coincidence output signalsfrom the AND gate are counted by a pulse counter which is automaticallyreset by a reset multivibrator if a pulse is missing. A given number ofpulses without a missing pulse will fire a Schmitt level detector whichcharges a charging network when the level deflector is in its on state.After a predetermined time the voltage across the charging network willtrigger a gated pulse generator which generates the alarm output pulse.If, however, a coincidence pulse is missing before the charging networkhas reached the firing voltage of the gated pulse generator the resetmultivibrators will operate to turn the Schmitt level detector to theoff state which will discharge the charging network. Thus the inputsignal of predetermined frequency must be present at the input for apredetermined minimum time before an output pulse will be generated.

In the drawings FIG. 1 is a representation of the overall alarm systemaccording to the invention.

FIG. 2 is a representation of the frequency diversity signals emitted bya single transmitter means of the invention.

FIG. 3 is a block diagram of a single transmitter means of theinvention.

FIG. 4 is a block diagram of a frequency diversity receiver means whichis located at a central station according to the invention.

FIG. 5 is a block diagram of a pulse rate discriminator means accordingto the invention.

FIG. 6 is a diagram of the waveforms produced by the pulse ratediscriminator means of FIG. 5.

An alarm system according to the present invention is illustrated inFIG. 1. In FIG. 1 receiver means 1 is positioned at a central locationin the alarm system installation. Sensing units 1 to 3 are units ofremotely positioned alarm condition sensing means, unit 1 beingcomprised in the diagram of FIG. 1 of sensing means 1a, lb and 1c, unit2 being comprised of sensing means 2a, 2b and 2c and unit 3 beingcomprised of sensing means 3a, 3b and 3c. Each sensing means such as lais comprised of one alarm condition sensing element such as an ionchamber or mechanical switch connected to one frequency diversity radiotransmitter means.

The transmitter means of sensing units 1, 2 and 3 emit frequencydiversity radio signals having the same carrier frequencies but thesignals emitted by the transmitter means of each different unit havedifferent modulation signals thereon as will be further explained below.The spectrum of a typical pair of frequency diversity signals emitted bya transmitter means of the present invention is shown in FIG. 2. Thecenter carrier frequency of the signals shown in FIG. 2 is 300 mhz witheach of the sidebands being displaced from the center carrier frequencyby 4.7 mhz. The modulation on each of the sidebands is identical.

All of the transmitter means of sensing units 1, 2 and 3 thus transmiton the same carrier frequencies, for instance at 295.7 mhz and 304.7 mhzif the illustrative frequency spectrum of FIG. 2 is used. Each of thepairs of frequency diversity signals associated with transmitter meansin different units however have different modulation frequenciesimpressed thereon.

Receiver means 1 is tuned to the carrier frequencies of the frequencydiversity signals emitted by the transmitter means of units 1, 2 and 3.The frequency diversity signals are demodulated in the receiver meansand each of the decoder means I, 2 and 3 is responsive to a demodulatedsignal of different frequency to produce an alarm output signal whichidentifies the sensing unit which has triggered. Thus, for instance, ifa sensing element of a sensing means of sensing unit 1 has beentriggered by an alarm condition to which the sensing element isresponsive it will trigger the transmitter means to which it isconnected which will transmit a signal to the receiver means at thecentral location. The signal will be demodulated by the receiver meansand the demodulated signal will operate decoder 1 which will trigger analarm signal. Corresponsindly, if a transmitter in a sensing means ofunits 2 or 3 is triggered the signals transmitted will operate decoders2 or 3 respectively to produce an alarm output signal. It should benoted that while for purposes of convenience only three sensing units,each having three sensing means therein, have been illustrated in anactual installation a great many more than three sensing units and threesensing means per sensing unit may be used. The only requirement is thatthe number of decoder means at the central receiving location be equalto the number of sensing units.

In the preferred embodiment of the invention all of the sensing elementsof each sensing unit are responsive to the same alarm condition whileeach sensing means of the same unit is located in a different remoteposition, for instance in different rooms in a building which is beingmonitored. Thus, in FIG. 1 sensing means 10, lb and 10 would all beresponsive to the same alarm condition while being located in differentremote locations. The sensing elements of unit 1 could, for instance, bethermistors if the condition sensed was to be temperature, and each ofthe sensing means la, lb, 1c would be located in a different room of abuilding being monitored with the receiver means 1 being located at acentral point in the building. In such an embodiment the sensingelements of units 2 and 3 would be different from the sensing elementsof unit I as well as being different from each other and the sensingmeans 2a, 2b, would be at different remote locations as would be thesensing means 30, 3b, and 3c. The sensing elements of unit 2 could forinstance be mechanical or magnetic switches attached to a door whichwhen opened would indicate the presence of an intruder. The sensingelements of unit 3 could be responsive to yet another condition or inthe alternative sensing means 3 could be hand held transmitting unitsoperated by persons such as security guards statione throughout thebuilding being monitored. A signal received at the central receivingstation from one of these transmitting units could indicate that help isneeded by one of the security guards. In the preferred embodiment thenan output signal emitted by decoder means 1, for instance, indicatesthat the alarm condition to which sensing unit 1 is responsive hastriggered one of the sensing means la, lb or Is at one of the remotelocations.

In another embodiment of the invention instead of the sensing elementsof each unit being responsive to the same condition but beingindifferent remote locations the sensing elements of each unit could berespon-. sive to different conditions but would be in the sameremotelocation. Thus, in this embodiment the sensing elements associatedwith sensing means la, lb, and 10 would each be different from the otherbut the sensing means of unit 1 would all be in the same remotelocation, for instance, in the same room in a building being monitored.In this embodiment then, different decoder outputs at the receivingstation would be indicative of trouble at a particular remote locationinstead of trouble with a particular condition being monitored at atleast one of the remote locations.

If desired, according to the present invention, another alarm systemcould be situated in close proximity to the first alarm system withoutinterfering therewith. This second alaram system is illustrated in FIG.I as comprising condition sensing units 4, 5 and 6 located at remotepositionsand receiver means 2 located at a central location. Thecondition sensing units and receiver means of the second system would beidentical to the first except that the transmitter means of units 4, 5and 6 and receiver means 2 would operate on carrier frequenciesdisplaced from the carrier frequencies used by the transmitter means ofunits 1, 2 and 3 and receiver means 1. The modulation frequencies on thesignals generated by the transmitter means of units 4, 5 and 6 wouldeach be difierent from the other, although they could be the same as themodulation frequencies on the transmitter means of units 1, 2 and 3, andwould trigger alarm outputs at decoders 4, 5 and 6. Because of thedifferent carrier frequencies used in the different alarm systems andthe extreme selectivity of the receiver means as will be elaborated onlater, interference between the two alarm systems is avoided. Hence, ina typical application one alarm system may be used to monitor onebuilding in a complex of buildings whereas the other alarm system couldbe used to monitor another building in that complex.

While each unit of sensing means has been described as having only threesensing means for purposes of convenience it is to be understood that ina typical installation each sensing unit may have a great many sensingmeans. For instance, in the preferred embodiment where each of thesensing means of each unit are responsive to the same condition one,sensing means from each unit might be positioned in a different room ina large building. Likewise while each system has been shown as beingcomprised of only three sensing units more than three units may beemployed in a practical system. And while only two alarm systemsoperating in proximity to each other have been illustrated in FIG. 1 itis apparent that many more than two systems may be used in the samevicinity where a large complex of buildings is monitored.

According to the FCC rules and regulations the band available foroperation is 285 to 328.6 mhz, and all RF channels of different systemsoperating in the same vicinity lie within this range. An importantfeature of the present invention is the use of frequency diversitytransmittter means and receiver means to provide extreme reliability andto eliminate multipath cancellation and fading. A transmitter means usedby the invention to generate a frequency diversity spectrum such as isshown In FIG. 2 is illustrated in block diagram in FIG. 3.

The transmitter means of each unit generate frequency diversity signalshaving a different modulation thereon. This modulation is provided byrelaxation oscillator 10 as shown in FIG. 3. This oscillator generates amodulation signal in the ultrasonic range which is used to amplitudemodulate subcarrier oscillator 11 which is an oscillator preferably ofColpitts design which is used to generate the subcarrier signal. If thetransmitter means of FIG. 3 is used to generate the i1- lustrativespectrum of FIG. 2 the subcarrier signal would be a signal of 4.7 mhz.

The RF oscillator was chosen to be a crystal oscillator to attain thedesired stability. A X3 multiplier is used to multiply the signalattained with the crystal oscillator to obtain an RF frequency withinthe FCC approved range. If the transmitter means were used to generatethe illustrative spectrum of FIG. 2 the RF oscillator and X3 multiplierwould generate a signal of 300 mhz. The outputs of the multiplier andthe subcarrier oscillator are mixed in a balanced modulator whichgenerates a spectrum as shown in FIG. 2. Antenna 15 may preferably be amonopole antenna which may be positioned on a printed circuit board onwhich the transmitter is built.

Amplitude modulation was found to be superior to frequency modulationfor the radio alarm links because it eliminated the triggering of falsealarms due to interference from local FM radio stations. Subcarrieroscillator 11 is amplitude modulated by the output of relaxationoscillator 10 by any standard technique of amplitude modulation such asis well known in the art. Additionally, each block shown in FIG. 3 is astandard electrical network as is well known in the art.

As shown in FIG. 1, receiver means 1 is preceded by a preselector filterto reject signals in the FM and TV broadcast band. This filter shouldprovide significant attenuation to signals below 250 mhz and above 350mhz and preferably may be a filter of ten pole Butterworth design. Thepreselector filter is an important component for rejecting unwantedsignals before they are processed by the receiver.

FIG. 4 is a block diagram of frequency diversity receiver means 1.Following front end bandpass filter 21 the receiver means has onechannel comprised of RF amplifier 22, local oscillator 25 and mixer 26,and a second channel comprised of RF amplifier 23, local oscillator 28,and mixer 27. The local oscillators are crystal controlled to achievethe desired stability. RF amplifier 22 is tuned to pass one of the sidebands of FIG. 2, for instance the lower side band, whereas RF amplifier23 would be tuned to pass the upper side band. Alternatively each RFamplifier could be tuned to pass both side bands. Local oscillator 25 isadjusted to generate a signal of a frequency which when mixed with thefrequency of the lower side band of the frequency diversity signal willresult in a signal having the intermediate frequency of the receivermeans. In the preferred embodiment of the invention the intermediatefrequency used is 10.7 mhz. Likewise local oscillator 28 is adjusted togenerate a signal which when mixed with a frequency of the upper sideband will result in a frequency of the intermediate frequency. A powerswitch switches between the two channels at a rate of about 4 cycles persecond. After being filtered in narrow-band filter 29 the intermediatefrequency signal is amplified in IF amplifier 30 and demodulated indemodulator 31. Demodulator 31 may be any standard demodulation devicefor demodulating an amplitude modulated signal.

The output of demodulator 31 is connected to decoders l, 2 and 3 asshown in FIG. 1. The purpose of the decoders are to selectively triggeran alarm signal responsive to the modulation on the transmittedfrequency diversity signals. Thus, in FIG. 1 decoder 1 will provide analarm output signal when a transmitter means of unit 1 has triggered.Likewise decoders 2 and 3 will emit alarm signals when activatedrespectively by transmitter means of units 2 and 3. The decoders are animportant part of the alarm system because the accuracy with which theydetect the ultrasonic frequency of the demodulated signals ensuresagainst the alarm system being triggered by false alarms. The decoderstake the form of pulse rate discriminator means which respond only tothe rate of zero crossings of the demodulated output signal. Hence anydependence upon the amplitude of the demodulated signal or on theamplitude of any interference signal is eliminated. Whereas bandpassfilters were found to respond to noise and to harmonics of the alarmsignal if they were of sufficient amplitude as well as to largeamplitude broadband noise such as generated by SCR controlled, variablespeed electric power tools, no such false alarms were triggered when thepulse rate discriminator means were used.

A block diagram of a pulse rate discriminator according to the inventionis shown in FIG. and waveforms associated therewith are shown in FIG. 6.The bandpass filter 40 at the input serves to reduce the noise which maybe present on the input signal. The output of bandpass filter 40 isshown in FIG. 6 at l and is a sine wave signal of period T. Schmitttrigger 41 acts as a squaring means on signal 1 to produce the waveformshown at 2 in FIG. 6 which waveform has a negative going edge every timewaveform 1 has a negative going axis crossing point. Pulse shaper 42generates a pulse each time that waveform 2 has a negative going edge.Thus waveform 3 shown in FIG. 6 consists of a series of pulses withperiod T. One output from pulse shaper 42 is fed to AND gate 45 and theother output is fed to resettable delay one shot multivibrator 43 whichgenerates a delay pulse as shown at 4 in FIG. 6. The resettable delayone-shot 43 operates only if the input pulse rate is less than somepreset maximum value.

The duration of delay pulse 43 is a substantial fraction of the durationT. In the preferred embodiment of the invention this fraction isapproximately nine-tenths. The lagging edge of waveform 4 triggers pulseone shot multivibrator 44 which generates a pulse of duration equal to asmall fraction of the period T. In the preferred embodiment of theinvention this fraction is approximately two-tenths.

The other input to the AND gate is the output from pulse oneshotmultivibrator 44. If the frequency to which the decoder is responsive ispresent at the input wave then the pulse generated by pulse shaper 42immediately following the pulse which has triggered delay one-shotmultivibrator 43 will coincide with the pulse output of pulse one-shot44. Thus as shown in FIG. 6 when the proper frequency is present at theinput, pulse 2 of pulse wave 3 coincides with pulse output 5 of pulseone-shot multivibrator 44. The coincidence or noncoincidence of pulse 2with waveform 5 is determined by AND gate 45.

Pulse counter 46 generates a ramp output the final amplitude of which isdetermined by the number of pulses counted. When the amplitude of theoutput of pulse counter 46 reaches a predetermined level Schmitt leveldetector 47 is triggered into its on state. Schmitt level detector 47 isa standard two state Schmitt device. Reset one-shot multivibrator 49acts as a missing pulse detector. Thus it is predetermined that a givennumber of successive outputs from AND gate 45 without a missing pulse isto be indicative that the proper frequency is present. This given numberof outputs from AND gate 45 is the number which will bring the output ofpulse counter 46 to the level at which Schmitt level detector 47 istriggered to its on state. If, however, a pulse is missing from thesuccessive train of pulses then reset one-shot multivibrator 49 istriggered which dumps the count of pulse counter 46 to zero and it muststart counting all over again to indicate the presence of the properfrequency.

According to one feature of the pulse rate discriminator a signal havingthe proper frequency must be present for at least milliseconds before anoutput alarm signal will be generated. Hence, when the output of pulsecounter 46 reaches a predetermined amplitude Schmitt level detector 47is turned on. The Schmitt detector will remain in its on state so longas pulse counter 46 keeps on receiving coincidence pulses from AND gate45 without a missing pulse. As long as Schmitt detector 49 is in its onstate charging network 49 charges. At the end of 100 millisecondsnetwork 49 which may be a simple RC network is charged to the pointwhere gated pulse generator 50 to which the RC network is connected,triggers its alarm output signal. If, however, a successive pulse ismissing during the 100 milliseconds reset one-shot multivibrator 49 willbe operative to return Schmitt level detector 47 to its off state. Whenthis happens charging network 49 discharges and a new 100 millisecondcriterion must be met. Thus the alarm output signal will not begenerated unless an input wave of proper brequency is present at theinput terminals for at least 100 milliseconds.

While I have described and illustrated a preferred embodiment of myinvention, I wish it to be understood that I do not intend to berestricted solely thereto, but that I do intend to cover allmodifications thereof which would be apparent to one skilled in the artand which come within the spirit and scope of my invention.

We claim: 1. An alarm system for indicating at a central receivinglocation which of a plurality of remotely located alarm conditionsensing units has triggered comprising,

a plurality of remotely located alarm condition sensing units, eachsensing unit being comprised of at least one sensing means which iscomprised of an alarm condition sensing element connected to a frequencydiversity radio transmitter means, each transmitter means beingoperative to emit a pair of frequency diversity radio signals when it istriggered by the sensing element to which it is connected, thetransmitter means of each unit including means for generating carriersignals of first and second predetermined frequencies and means formodulating said carrier signals with a modulation signal of a singlefixed frequency, the modulation signals produced by the transmittermeans of each unit being of a different fixed frequency, whereby thetransmitter means of each unit when triggered emit frequency diversitycarrier signals having the same carrier frequencies as the carriersignals emitted by the transmitter means of each other unit, whichcarrier signals are modulated with a different fixed modulationfrequency than the carrier signals emitted by the transmitter means ofeach other unit and a frequency diversity receiver means at said centralreceiving location including means for selectively passing said firstand second predetermined frequencies and further including ademodulation means for demodulating said frequency diversity signals toproduce demodulated signals of said different fixed modulationfrequencies, and decoder means responsive to said fixed frequencysignals for producing a different alarm output signal for each of saiddifferent frequencies whereby the alarm condition sensing unit which hastriggered is identified, said decoder means comprising a plurality ofpulse rate discriminator means, equal in number to the number of sensingunits, each pulse rate discriminator means being responsive to adifferent one of said fixed frequency signals for producing a differentalarm output signal.

2. The system of claim 1 wherein each of said pulse rate discriminatormeans includes means responsive to the rate of zero axis crossings ofsaid demodulated fixed frequency signals.

3. The alarm system of claim 1 wherein each of said sensing units iscomprised of a plurality of said sensing means.

4. The alarm system of claim 3 wherein the sensing elements of eachsensing unit are responsive to a different condition from the sensingelements of each other sensing unit and wherein each sensing element ofa single sensing unit is positioned at a different remote location fromthe other sensing elements in that unit.

5. The alarm system of claim 3 wherein the sensing elements of eachsensing unit are positioned at a different remote location from thesensing elements of each other unit and wherein each sensing element ofa single unit is responsive to a different condition than the othersensing elements in that unit.

one sensing element of each sensing unit is positioned at a differentremote location.

8. The alarm system of claim 1 wherein said receiver means furtherincludes means for producing said different alarm output signals whenonly one of any pair of frequency diversity signals is received at saidreceiver means.

9. The alarm system of claim 1 wherein the signals of each pair offrequency diversity signals are spaced from each other in the frequencyspectrum by a small amount compared to the carrier frequency of eitherof said signals.

10. The alarm system of claim 9 wherein said means for generatingcarrier signals of first and second predetermined frequencies includes asubcarrier oscillator means for generating a subcarrier signal having afrequency of half said small amount, a crystal controlled oscillatormeans for generating a center carrier frequency signal having afrequency midway between said first and second predetermined frequenciesand a balanced modulator means connected to said subcarrier oscillatormeans and to said crystal oscillator means for combining said subcarriersignal and said center carrier frequency signal. v

11. The alarm system of claim 10 wherein said means for modulating saidcarrier signals comprises a means for amplitude modulating said signalgenerated by said subcarrier oscillator means.

12. The alarm system of claim 11 wherein said single fixed frequency isa frequency in the ultrasonic range.

13. The alarm system of claim 12 wherein said means for amplitudemodulating is a relaxation oscillator and said subcarrier oscillatormeans is a Colpitts type oscillator.

M. The alarm system of claim 12 wherein said frequency diversityreceiver means at said central location is a two channel superhetrodynetype receiver means, each of said channels including means tuned toreceive one of said frequency diversity signals of a pair of saidfrequency diversity signals and local oscillator and mixer means forconverting said signals to an intermediate frequency signal, means forswitching between said local oscillator and mixer means of said twochannels, and a single intermediate frequency amplifier means foramplifying said intermediate frequency signal.

15. The system of claim 14 wherein each of said pulse rate discriminatormeans includes means responsive to the rate of zero crossings of saiddemodulated fixed frequency signals.

16. The alarm system of claim 9 wherein said frequency diversityreceiver means at said central location is a two channel superhetrodynetype receiver means, each of said channels including means'tuned toreceive one of said frequency diversity signals of a pair of saidfrequency diversity signals and local oscillator and mixer means forconverting said signals to an intermediate frequency signal, means forswitching between said local oscillator and mixer means of said twochannels, and a single intermediate frequency amplifier means foramplifying said intermediate frequency signal.

17. The system of claim 16 wherein each of said tuned means comprises aradio frequency amplifier means.

18. The system of claim 17 further including a preselector filter meansconnected to the input of said receiver means.

19. The alarm system of claim 1 wherein each pulse rate discriminatormeans includes means for generating a pulse of duration equal to a fixedfraction of the period corresponding to a given one of said modulationfrequencies.

20. The alarm system of claim 19 wherein said means for generatingcarrier signals of first and second predetermined frequencies includes asubcarrier oscillator means for generating a subcarrier signal having afrequency of half said small amount, a crystal controlled oscillatormeans for generating a center carrier frequency signal having afrequency midway between said first and second predeterminedfrequencies, and a balanced modulator means connected to said subcarrieroscillator means to said crystal oscillator means for combining saidsubcarrier signal and said center carrier frequency signal, and whereinsaid means for modulating said carrier signals comprises means foramplitude modulating said signal generated by said subcarrier oscillatormeans and wherein said frequency diversity receiver means at saidcentral location is a dual channel superhetrodyne type receiver means,each of said channels including means tuned to receive one of saidfrequency diversity signals of a pair of frequency diversity signals,and means for switching between said channels.

21. The system of claim 1 wherein said means for modulating said carriersignals comprises a means for generating a fixed frequency signal in theultrasonic range.

22, An alarm system comprising a plurality of central receivinglocations in the same vicinity, a group of alarm condition sensing unitsbeing associated with each central receiving location, each sensing unitbeing comprised of at least one sensing means which is comprised of analarm condition sensing element connected to a frequency diversitytransmitter means for generating a pair of frequency diversity signals,each of said transmitter means including first generating means forgenerating a center carrier frequency signal for said pair of frequencydiversity signals, the center carrier frequency signal generated by saidfirst generating means of the transmitter means of each group being ofthe same frequency but of different frequency that the signal generatedby said first generating means of the transmitter means of each group,each of said transmitter means further including second generating meansfor generating a fixed frequency modulation signal, the modulationsignal generated by said second generating means of the transmittermeans of each unit being of a different fixed frequency than themodulation signal generated by said second generating means of thetransmitter means of each other unit in the same group, each transmittermeans further including means for combining said carrier signals withsaid modulation signal whereby each transmitter means when triggeredemits a pair of frequency diversity signals having the same centercarrier frequency as the signals emitted by each other transmittermeansin the same group but a different center carrier frequency than thesignals emitted by transmitter means in each other group and having afixed frequency modulation signal of different frequency than themodulation signal of the signals emitted by transmitter means in otherunits of the same group,

and a plurality of frequency diversity receiver means at said pluralityof central receiving locations, neach receiver means including means forselectively passing signals of the center carrier frequency generated bysaid first generating means of the transmitter means of the group withwhich said receiver means is associated, and further including ademodulation means for demodulating said frequency diversity signals toproduce different fixed frequency demodulated signals, each receivermeans further including a plurality of pulse rate discriminator meansconnected to its output of a number equal to the number of sensing unitsof the group with which said receiver is associated, each pulse ratediscriminator means being responsive to a signal of different frequencyto activate a different alarm output, whereby the particular alarmoutput activated is indicative of both the group and unit of thetransmitter means which has triggered.

1. An alarm system for indicating at a central receiving location whichof a plurality of remotely located alarm condition sensing units hastriggered comprising, a plurality of remotely located alarm conditionsensing units, each sensing unit being comprised of at least one sensingmeans which is comprised of an alarm condition sensing element connectedto a frequency diversity radio transmitter means, each transmitter meansbeing operative to emit a pair of frequency diversity radio signals whenit is triggered by the sensing element to which it is connected, thetransmitter means of each unit including means for generating carriersignals of first and second predetermined frequencies and means formodulating said carrier signals with a modulation signal of a singlefixed frequency, the modulation signals produced by the transmittermeans of each unit being of a different fixed frequency, whereby thetransmitter means of each unit when triggered emit frequency diversitycarrier signals having the same carrier frequencies as the carriersignals emitted by the transmitter means of each other unit, whichcarrier signals are modulated with a different fixed modulationfrequency than the carrier signals emitted by the transmitter means ofeach other unit and a frequency Diversity receiver means at said centralreceiving location including means for selectively passing said firstand second predetermined frequencies and further including ademodulation means for demodulating said frequency diversity signals toproduce demodulated signals of said different fixed modulationfrequencies, and decoder means responsive to said fixed frequencysignals for producing a different alarm output signal for each of saiddifferent frequencies whereby the alarm condition sensing unit which hastriggered is identified, said decoder means comprising a plurality ofpulse rate discriminator means, equal in number to the number of sensingunits, each pulse rate discriminator means being responsive to adifferent one of said fixed frequency signals for producing a differentalarm output signal.
 2. The system of claim 1 wherein each of said pulserate discriminator means includes means responsive to the rate of zeroaxis crossings of said demodulated fixed frequency signals.
 3. The alarmsystem of claim 1 wherein each of said sensing units is comprised of aplurality of said sensing means.
 4. The alarm system of claim 3 whereinthe sensing elements of each sensing unit are responsive to a differentcondition from the sensing elements of each other sensing unit andwherein each sensing element of a single sensing unit is positioned at adifferent remote location from the other sensing elements in that unit.5. The alarm system of claim 3 wherein the sensing elements of eachsensing unit are positioned at a different remote location from thesensing elements of each other unit and wherein each sensing element ofa single unit is responsive to a different condition than the othersensing elements in that unit.
 6. The alarm system of claim 1 whereinthe at least one sensing element of each sensing unit is responsive to adifferent condition.
 7. The alarm system of claim 1 wherein the at leastone sensing element of each sensing unit is positioned at a differentremote location.
 8. The alarm system of claim 1 wherein said receivermeans further includes means for producing said different alarm outputsignals when only one of any pair of frequency diversity signals isreceived at said receiver means.
 9. The alarm system of claim 1 whereinthe signals of each pair of frequency diversity signals are spaced fromeach other in the frequency spectrum by a small amount compared to thecarrier frequency of either of said signals.
 10. The alarm system ofclaim 9 wherein said means for generating carrier signals of first andsecond predetermined frequencies includes a subcarrier oscillator meansfor generating a subcarrier signal having a frequency of half said smallamount, a crystal controlled oscillator means for generating a centercarrier frequency signal having a frequency midway between said firstand second predetermined frequencies and a balanced modulator meansconnected to said subcarrier oscillator means and to said crystaloscillator means for combining said subcarrier signal and said centercarrier frequency signal.
 11. The alarm system of claim 10 wherein saidmeans for modulating said carrier signals comprises a means foramplitude modulating said signal generated by said subcarrier oscillatormeans.
 12. The alarm system of claim 11 wherein said single fixedfrequency is a frequency in the ultrasonic range.
 13. The alarm systemof claim 12 wherein said means for amplitude modulating is a relaxationoscillator and said subcarrier oscillator means is a Colpitts typeoscillator.
 14. The alarm system of claim 12 wherein said frequencydiversity receiver means at said central location is a two channelsuperhetrodyne type receiver means, each of said channels includingmeans tuned to receive one of said frequency diversity signals of a pairof said frequency diversity signals and local oscillator and mixer meansfor converting said signals to an intermediate frequency signal, meansfor switching between said local oscillator and mixer Means of said twochannels, and a single intermediate frequency amplifier means foramplifying said intermediate frequency signal.
 15. The system of claim14 wherein each of said pulse rate discriminator means includes meansresponsive to the rate of zero crossings of said demodulated fixedfrequency signals.
 16. The alarm system of claim 9 wherein saidfrequency diversity receiver means at said central location is a twochannel superhetrodyne type receiver means, each of said channelsincluding means tuned to receive one of said frequency diversity signalsof a pair of said frequency diversity signals and local oscillator andmixer means for converting said signals to an intermediate frequencysignal, means for switching between said local oscillator and mixermeans of said two channels, and a single intermediate frequencyamplifier means for amplifying said intermediate frequency signal. 17.The system of claim 16 wherein each of said tuned means comprises aradio frequency amplifier means.
 18. The system of claim 17 furtherincluding a preselector filter means connected to the input of saidreceiver means.
 19. The alarm system of claim 1 wherein each pulse ratediscriminator means includes means for generating a pulse of durationequal to a fixed fraction of the period corresponding to a given one ofsaid modulation frequencies.
 20. The alarm system of claim 19 whereinsaid means for generating carrier signals of first and secondpredetermined frequencies includes a subcarrier oscillator means forgenerating a subcarrier signal having a frequency of half said smallamount, a crystal controlled oscillator means for generating a centercarrier frequency signal having a frequency midway between said firstand second predetermined frequencies, and a balanced modulator meansconnected to said subcarrier oscillator means to said crystal oscillatormeans for combining said subcarrier signal and said center carrierfrequency signal, and wherein said means for modulating said carriersignals comprises means for amplitude modulating said signal generatedby said subcarrier oscillator means and wherein said frequency diversityreceiver means at said central location is a dual channel superhetrodynetype receiver means, each of said channels including means tuned toreceive one of said frequency diversity signals of a pair of frequencydiversity signals, and means for switching between said channels. 21.The system of claim 1 wherein said means for modulating said carriersignals comprises a means for generating a fixed frequency signal in theultrasonic range.
 22. An alarm system comprising a plurality of centralreceiving locations in the same vicinity, a group of alarm conditionsensing units being associated with each central receiving location,each sensing unit being comprised of at least one sensing means which iscomprised of an alarm condition sensing element connected to a frequencydiversity transmitter means for generating a pair of frequency diversitysignals, each of said transmitter means including first generating meansfor generating a center carrier frequency signal for said pair offrequency diversity signals, the center carrier frequency signalgenerated by said first generating means of the transmitter means ofeach group being of the same frequency but of different frequency thatthe signal generated by said first generating means of the transmittermeans of each group, each of said transmitter means further includingsecond generating means for generating a fixed frequency modulationsignal, the modulation signal generated by said second generating meansof the transmitter means of each unit being of a different fixedfrequency than the modulation signal generated by said second generatingmeans of the transmitter means of each other unit in the same group,each transmitter means further including means for combining saidcarrier signals with said modulation signal whereby each transmittermeans when triggered emits a pair of frequency dIversity signals havingthe same center carrier frequency as the signals emitted by each othertransmitter means in the same group but a different center carrierfrequency than the signals emitted by transmitter means in each othergroup and having a fixed frequency modulation signal of differentfrequency than the modulation signal of the signals emitted bytransmitter means in other units of the same group, and a plurality offrequency diversity receiver means at said plurality of centralreceiving locations, neach receiver means including means forselectively passing signals of the center carrier frequency generated bysaid first generating means of the transmitter means of the group withwhich said receiver means is associated, and further including ademodulation means for demodulating said frequency diversity signals toproduce different fixed frequency demodulated signals, each receivermeans further including a plurality of pulse rate discriminator meansconnected to its output of a number equal to the number of sensing unitsof the group with which said receiver is associated, each pulse ratediscriminator means being responsive to a signal of different frequencyto activate a different alarm output, whereby the particular alarmoutput activated is indicative of both the group and unit of thetransmitter means which has triggered.